Some planetary-mass objects are thought to have formed in a similar way to stars, and the IAU has proposed that those objects be called sub-brown dwarfs[4] (an example of this is Cha 110913-773444, which may be an ejected rogue planet or may have formed on its own and be a sub-brown dwarf).[5] The closest free-floating planetary mass object to Earth yet discovered, WISE 0855–0714, is around 7 light years away.

In December 2013 a candidate exomoon of a free-floating planet was announced.[6]

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Observation[edit]

Most methods of detecting exoplanets rely on periodicity of the planet orbiting a host star and thus cannot be used to detect rogue planets. Two methods to detect rogue planets still can be used: gravitational microlensing and direct imaging.

Direct imaging allows astronomers to observe rogue planets continuously. However, only young and massive rogue planets can be observed this way because they emit enough radiation to be detected. On the other hand, without the glare of the host star, the planet itself can be observed more easily once found.

When a planetary-sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. The disadvantage of microlensing is that the planet cannot be continuously observed. However, it works better than direct imaging for older and lower-mass planets. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published in 2011. They observed 50 million stars in our galaxy using the 1.8 meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3 meter University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two free-floaters for every star in our galaxy.[7][8][9] Other estimations suggest a much larger number, up to 100,000 times more free-floating planets than stars in our Milky Way.[10] In November 2012 astronomers discovered a rogue planet around 100 light-years from Earth.[11]

Retention of heat in interstellar space[edit]

In 1998, David J. Stevenson theorized[12] that some planet-sized objects drift in the vast expanses of cold interstellar space and could possibly sustain a thick atmosphere that would not freeze out. He proposes that atmospheres are preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.

It is thought that, during planetary-system formation, several small protoplanetary bodies may be ejected from the forming system.[13] With the reduced ultraviolet light that would normally strip the lighter components from an atmosphere, due to its increasing distance from the parent star, the planet's predominantly hydrogen- and helium-containing atmosphere would be easily confined even by an Earth-sized body's gravity.[12]

It is calculated that, for an Earth-sized object at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures above the melting point of water.[12] Thus, it is proposed that interstellar planetary bodies with extensive liquid-water oceans may exist. It is further suggested that these planets are likely to remain geologically active for long periods, providing a geodynamo-created protective magnetosphere and possible sea floorvolcanism which could provide an energy source for life.[12] Thus humans could theoretically live on a planet without a sun, although food sources would be limited. The author admits these bodies would be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere, although later research suggests[14] that reflected solar radiation and far-IR thermal emissions may be detectable if one were to pass within 1000 AU of Earth.

A study of simulated planet ejection scenarios has suggested that around five percent of Earth-sized planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating.[15]

Proplyds of planetars[edit]

Recently, it has been discovered that some exoplanets such as the planemo2M1207b, orbiting the brown dwarf 2M1207, have debris discs. If some large interstellar objects are considered stars (sub-brown dwarfs), then the debris could coalesce into planets, meaning the disks are proplyds. If these are considered planets, then the debris would coalesce as satellites. The term planetar exists for those accretion masses that seem to fall between stars and planets.

Known or possible rogue planets[edit]

There is no current way of telling whether these are planets that have been ejected from orbiting a star or were originally formed on their own as sub-brown dwarfs.